Butyl elastomeric compositions having reduced permeability to gases

ABSTRACT

A new rubber composition, the vulcanizate of which has reduced permeability to gases, comprises: one hundred parts by weight of at least one butyl-type rubbery polymer; from about 3 to about 20 parts by weight of α-methylstyrene homopolymer having a softening point of from about 93° C. to about 150° C. and a Tg of from about 15° C. to about 75° C.; from about 30 to about 90 parts by weight of at least one carbon black, from 0 to about 7 parts by weight of hydrocarbon extender oil; and a curing system. A process for producing said rubber composition is also provided.

FIELD OF THE INVENTION

This invention relates to synthetic rubber compositions and a processfor producing vulcanizates thereof. More particularly, the inventionrelates to elastomeric compositions comprising mixtures of butyl-typeelastomeric polymers and α-methylstyrene homopolymers, and a process forproducing vulcanizates thereof, for use in articles requiring low orreduced permeability to gases.

BACKGROUND OF THE INVENTION

Butyl elastomeric polymers have been known and commercially availablefor many years. They possess a variety of inherently satisfactoryproperties as elastomers which has enabled them to find utility in manycommercial areas. Among their satisfactory inherent properties are theirimpermeability to air, high damping of low frequency vibrations, andgood resistance to aging, heat, acids, bases, ozones and other chemicalsafter vulcanization, which renders them well suited for use in a varietyof applications including articles requiring low or reduced permeabilityto air. Examples of such articles include, but are not limited to, tireinner tubes, tire curing bladders and various air bladders.

Halogenated butyl elastomeric polymers have also been known andcommercially available for many years. In addition to possessing thesatisfactory inherent properties of butyl elastomeric polymers describedabove, halogenated butyl elastomeric polymers also possess curecompatibility with more highly unsaturated rubbers and good adhesion tosuch other rubbers after vulcanization, which renders them well suitedfor use in pneumatic tire inner liners. In the tire industry'scontinuing quest for improved inner liners, an elastomeric compoundwhich exhibits both an improvement in processability and, aftervulcanization, an improvement in air impermeability over conventionalinner liners would be desirable, provided that these improvements alsoresulted in the maintenance of a high level of strength and adhesiveproperties.

DESCRIPTION OF PRIOR ART

U.S. Pat. No. 4,487,892 (Ohmori et al) discloses rubbery compositionsfor use in tire treads comprising blends of resinous polymers (1 to 30parts by weight) having Tg of not less than 110° C. with at least onerubber (70 to 99 parts by weight) selected from the group consisting ofstyrene-butadiene copolymer rubber, polybutadiene rubber, butyl rubber,halogenated butyl rubber and polyisoprene rubber. The blends may be madeby mastication on a mill or in an internal mixer.

U.S. Pat. No. 4,328,133 (Ogawa et al) discloses rubbery compositionswhich have been reinforced with organic micro fibers having Tg lowerthan 30° C. or higher than 120° C. and a specified average length,average diameter and ratio of the average diameter to the averagelength. The organic micro fibers are added in the amount 3-30 parts byweight based on 100 parts by weight of at least one rubber selected fromthe group consisting of natural rubber, synthetic polyisoprene rubber,butyl rubber, halogenated butyl rubber, polybutadiene rubber,styrene-butadiene copolymer rubber, ethylene-propylenediene terpolymerrubber and acrylonitrile-butadiene copolymer rubber.

U.S. Pat. No. 4,396,051 (Ogawa et al) discloses the use of variousorganic micro fibers described in U.S. Pat. No. 4,328,133 in the innerliner of a pneumatic tire. The rubber blend suitable for use comprisesat least 50 percent by weight of butyl rubber and/or halogenated butylrubber, and at most 50 percent by weight of a diene rubber. The authorsdisclose that when such a rubber blend is reinforced with organic microfibers. vulcanized and formed into an inner liner. the result will be apneumatic tire having low air permeability, improved fatigue resistanceand low fuel consumption properties.

SUMMARY OF THE INVENTION

It is an objective of this invention to provide a new rubber compositionhaving improved processability characteristics, which composition, aftervulcanization, possesses very low permeability to gases, while alsomaintaining a high level of strength and adhesive properties.

It is a further objective of this invention to provide a process forproducing a rubber vulcanizate having reduced permeability to gases.

It has now been found, both surprisingly and unexpectedly, that rubbercompositions having improved processability characteristics and whichare sulphur vulcanizable to produce tire inner liners, air bladders andsimilar compounds having a reduced permeability to gases, and adesirable balance of strength and adhesion properties, can be preparedby mixing together, in suitable proportions, a butyl-type rubber andα-methylstyrene homopolymer having a softening point of from about 93°C. to about 150° C. and a glass transition temperature (Tg) of fromabout 15° to about 75° C.

Accordingly, in one aspect, our invention provides a new rubbercomposition, the vulcanizate of which has reduced permeability to gases,comprising

one hundred parts by weight of at least one rubbery polymer selectedfrom the group consisting of butyl rubber, chlorinated butyl rubber andbrominated butyl rubber,

from about 3 to about 20 parts by weight of α-methylstyrene homopolymerhaving a softening point of from about 93° C. to about 150° C. and a Tgof from about 15° to about 75° C.

from about 30 to about 90 parts by weight of at least one carbon black,

from 0 to about 7 parts by weight of hydrocarbon extender oil, and

a curing system.

In another aspect, our invention provides a process for producing avulcanizate having reduced permeability to gases which comprises:

(A) mixing one hundred parts by weight of at least one rubbery polymerselected from the group consisting of butyl rubber, chlorinated butylrubber and brominated butyl rubber with an α-methylstyrene homopolymer,wherein said α-methylstyrene homopolymer is used in an amount of fromabout 3 to about 20 parts by weight and has a softening point of fromabout 93° C. to about 150° C. and a Tg of from about 15° C. to about 75°C. from about 30 to about 90 parts by weight of at least one carbonblack, from 0 to about 7 parts by weight of hydrocarbon extender oil,and a curing system,

(B) forming the mixture produced in (A) into a sheet, and

(C) vulcanizing the sheet formed in (B) at a temperature of from about150° C. to about 200° C. for from about 5 to about 60 minutes.

DETAILED DESCRIPTION OF THE INVENTION

Butyl rubber is well known in the art and is a polymer of a C₄ -C₆isoolefin, preferably isobutylene, and a C₄ -C₆ conjugated diolefin,preferably isoprene. A preferred butyl polymer contains from about 97 to99.5 weight percent of isobutylene and from about 0.5 to about 3 weightpercent of isoprene. Butyl polymer typically has a molecular weightexpressed as the Mooney (ML 1+8 at 125° C.), of from about 25 to about65, preferably from about 40 to about 60.

Halogenated butyl polymer is well known in the art and is selected frombromobutyl or chlorobutyl polymer; it is produced by bromination orchlorination of butyl polymer. Bromobutyl polymer typically containsfrom about 1 to about 3, preferably from about 1 to about 2, weightpercent of isoprene and from about 97 to about 99, preferably from about98 to about 99, weight percent of isobutylene, based on the hydrocarboncontent of the polymer, and from about 1 to about 4, preferably fromabout 1.5 to about 3, weight percent of bromine, based on the bromobutylpolymer. A typical bromobutyl polymer has a molecular weight, expressedas the Mooney viscosity (ML 1+8 at 125° C.), of from about 25 to about55. Chlorobutyl polymer typically contains from about 1 to about 3,preferably from about 1 to about 2, weight percent of isoprene and fromabout 97 to about 99, preferably from about 98 to about 99, weightpercent of isobutylene, based on the hydrocarbon content of the polymer,and from about 0.5 to about 2.5, preferably from about 0.75 to about1.75, weight percent of chlorine, based on the chlorobutyl polymer. Atypical chlorobutyl polymer has a molecular weight, expressed as theMooney viscosity (ML 1+8 at 125° C.), of from about 35 to about 55.

Polymers of α-methylstyrene are known commercially available products.One such polymer is sold under the tradename AMOCO and is available asone of Resin 18-210, Resin 18-240 or Resin 18-290. These polymers arebelieved to contain essentially α-methylstyrene homopolymer and belongto a class of resins having a softening point of from about 93° C. (200°F.) to about 150° C. (300° F.) and a Tg of from about 15° to about 75°C. The resinous homopolymers are generally available as pellets whichare white in colour.

The use of carbon blacks is well known in the art for reinforcement ofvulcanizates and results in improved strength properties of the finalvulcanizate. Suitable carbon blacks for practicing this invention arethe well known furnace and channel, preferably furnace, blacks and areused in the amounts of from about 30 to about 90, preferably from about50 to about 70, parts by weight.

The use of hydrocarbon extender oil is well known in the art; the use ofsuch extender oils in vulcanizates generally causes the air permeabilityof such vulcanizates to increase. Suitable hydrocarbon extender oilsinclude the paraffinic or naphthenic extender oils, preferablyparaffinic. and are used in amounts of from 0 to about 7, preferablyfrom 0 to about 4, parts by weight. In a most preferred embodiment, thecompositions contain no hydrocarbon extender oil.

The curing system suitable for use in the present invention is notparticularly restricted. A typical curing system comprises: (i) a metaloxide, (ii) elemental sulphur and (iii) at least one sulphur basedaccelerator. The use of metal oxides as a component in the curing systemis well known in the art. A suitable metal oxide is zinc oxide. which isused in amounts of from about 1 to about 10, preferably from about 2 toabout 5, parts by weight. Elemental sulphur, comprising component (ii)of said curing system is used in amounts of from about 0.2 to about 2parts by weight. Suitable sulphur based accelerators (component (iii) ofsaid curing system) are used in amounts of from about 0.5 to about 3parts by weight and may be selected from the thiuram sulphides such astetramethyl thiuram disulphide (TMTD), the thiocarbamates such as zincdimethyl dithiocarbamate (ZDC) and the thiazyl and benzothiazylcompounds such as mercaptobenzothiazyl disulphide (MBTS). Preferably,the sulphur based accelerator is mercaptobenzothiazyl disulphide.

Stabilizers, anti-oxidants and tackifiers may also be added in the usualway and in the normal amounts for compounding butyl-type rubberypolymers.

The procedure of mixing the various components of this invention is notspecifically restricted. In one embodiment, all of the ingredients ofthe rubber composition described above may be mechanically mixed at aninitial temperature of not more than about 80° C. in an internal mixerand then vulcanized in a conventional manner. In another embodiment, therubbery polymer and α-methylstyrene homopolymer may be solution blended,said blend recovered from solution prior to being mechanically mixedwith the remaining ingredients and then vulcanized in a conventionalmanner. Thus a blend of a rubbery polymer and α-methylstyrenehomopolymer is suitably achieved using solutions of the polymers inmutually compatible hydrocarbon liquid solvents. Such a blend may berecovered from solution by standard recovery techniques of solventremoval and drying, followed by the compounding and vulcanizationprocedure described above. In yet another embodiment, theα-methylstyrene homopolymer may be added, in bulk, to the rubberypolymer prior to the compounding operation, such as to the rubberypolymer during a final stage of the manufacturing process thereof.

Generally, it is preferred to carry out the compounding procedure in twostages. In the first stage the polymers may be mixed with conventionalcompounding ingredients; these may include carbon black, hydrocarbonextender oil, tackifiers, stabilizers, processing aids andanti-oxidants. In the second stage of the compounding procedure, thecure active agents are preferably added to the compound described aboveon a rubber mill or in an internal mixer operated at a temperaturenormally not in excess of about 60° C. The cure active agents mayinclude elemental sulphur, accelerators and zinc oxide. The compoundsare cured in a conventional manner by heating for from about 5 to about60 minutes at temperatures of from about 150° C. to about 200° C. toform elastomeric vulcanizates.

After vulcanization, the rubber compositions described above exhibit,both surprisingly and without scientific explanation, a markedimprovement in air impermeability while maintaining a desirable balanceof other physical properties. By an improvement in air impermeability ismeant a reduction in the volume of air that passes through a unit areaof defined thickness of the vulcanizate per unit of time. By physicalproperties is meant hardness, elongation, fatigue life, adhesion andstrength properties, which include modulus at 100 percent elongation,modulus at 300 percent elongation and tensile strength at rupture.Accordingly, said vulcanizates are suitable for use in articlesrequiring low or reduced permeabilty to gases while maintaining adesirable balance of other physical properties.

In preferred embodiments of the present invention, a rubber compositioncomprises: 100 parts by weight of halogenated butyl rubber; from about12 to about 18 parts by weight of α-methylstyrene homopolymer which hasa softening point of from about 93° to about 150° C., preferably fromabout 110° C. to about 150° C., and a Tg of from about 15° to about 75°C., preferably from about 50° C. to about 75° C.; from about 50 to about70 parts by weight of at least one carbon black; from 0 to about 4 partsby weight of a hydrocarbon extender oil; and a curing system. Avulcanizate derived from such a composition is suitable for use in theproduction of a rubber membrane material. Such a membrane material wouldhave many uses. These may include, but are not limited to, pneumatictire inner liners, aerosol spray can linings and air bladders used insoccer balls, footballs, basketballs, volleyballs, air mattresses andtennis balls.

In other preferred embodiments of the present invention, a rubbercomposition comprises: 100 parts by weight of brominated butyl rubber;from about 12 to about 18 parts by weight of α-methylstyrene homopolymerwhich has a softening point of from about 93° to about 150° C., mostpreferably from about 130° to about 150° C. and a Tg of from about 15°to about 75° C., most preferably from about 65° to about 75° C.; fromabout 50 to about 70 parts by weight of at least one carbon black., from0 to about 4. most preferably 0, parts by weight of a hydrocarbonextender oil; and a curing system. Such a composition exhibits animprovement in processability characteristics, which include millshrinkage, extrudability and green strength. Further, the vulcanizatederived from such a composition is characterized by having a markedreduction in air permeability and a marked increase in adhesion andthus, is suitable for use in a pneumatic tire inner liner application.

According to yet other preferred embodiments of the present invention aprocess for producing a vulcanizate having reduced permeability to gasescomprises:

(A) mixing one hundred parts by weight of at least one rubbery polymerselected from the group consisting of butyl rubber, chlorinated butylrubber and brominated butyl rubber, preferably brominated butyl rubber,with an α-methylstyrene homopolymer, wherein said α-methylstyrenehomopolymer is used in an amount of from about 12 to about 18 parts byweight and has a softening point of from about 93° C. to about 150 C.,more preferably from about 110° C. to about 150° C. and most preferablyfrom about 130° C. to about 150° C., and a Tg of about 15° C. to about75° C., more preferably from about 50° C. to about 75° C. and mostpreferably from about 50° C. to about 75° C., and most preferably fromabout 65° C. to about 75° C., from about 30 to about 90 parts by weightof at least one carbon black, from 0 to about 7 parts by weight ofhydrocarbon extender oil, and a curing system,

(B) forming the mixture produced in (A) into a sheet, and

(C) vulcanizing the sheet formed in (B) at a temperature of from about150° C. to about 200° C. for from about 5 to about 60 minutes.

The following examples illustrate the present invention and are notintended to limit the scope thereof. All parts are parts by weightunless otherwise specified. The materials used were:

Butyl rubber--commercial product sold as POLYSAR® Butyl 301

Brominated butyl rubber--commercial product sold as POLYSAR® Bromobutyl2030

Chlorinated butyl rubber--commercial product sold as POLYSAR®.Chlorobutyl 1240

α-Methylstyrene homopolymer--commercial product sold as AMOCO® Resin18-210 (softening point=210° F.), AMOC® Resin 18-240 (softeningpoint=240° F., AMOCO® Resin 18-290 (softening point=290° F.)

Pentalyn "A"--tackifier--pentaerythritol ester of rosin

Carbon black--commercial product N-660 type

Paraffinic Oil--commercial product sold as Sunpar 2280

MBTS (mercaptobenzothiazyl disulphide: accelerator), TMTD(tetramethylthiurarm disulphide: accelerator), sulphur (curative),stearic acid (activator and processing aid) and zinc oxide(vulcanization agent) were commercially available materials

EXAMPLE 1

Ten bromobutyl polymer compounds were prepared according to the generalrecipe shown in Table 1, in which the bromobutyl polymer is the same asthat described above. The glass transition temperature of theα-methylstyrene homopolymer used in the recipe was measured according toASTM D3418-82. Compounding was done by mixing in a Laboratory Banbury,Model "B" internal mixer. The starting temperature of the mixer was 40°C. and the mixing cycle was completed in four minutes. Compounding wascompleted by adding the remaining ingredients (sulphur and zinc oxide)on a two-roll rubbermill at 40° C. The compounded stock was formed intosheets and vulcanized for 30 minutes at 166° C.

Tensile strength, modulus at 100 percent and 300 percent elongation andultimate elongation to break were determined according to ASTM D412-80.Hardness was determined using a Type A Shore durometer according to ASTMD2240-81. Hot air aging was completed according to ASTM D573-81. Millshrinkage of the compounded rubber compositions was determined accordingto ASTM D1917-84.

The air impermeability of the vulcanizates was determined according tothe following procedure. A vulcanized rubber sheet was placed into apermeability test cell at which point a positive gas pressure of 50 psiwas applied to one side of the sheet. The test cell was thenpreconditioned for 24 hours at room temperature to allow the system toestablish an equilibrium prior to actual testing, after which the testcell was mounted in a constant temperature bath at 65° C. The gaspermeating the specimen displaced a liquid from a graduated capillarytube, permitting a direct measurement of the volume. Accordingly, thereported value for air impermeability represents the volume of airpassing, per second, through a specimen of 1 cm² area and 1 cm thicknesswhen the difference in pressure across the specimen is one atmosphere.

                  TABLE I                                                         ______________________________________                                                          Parts by weight                                             ______________________________________                                        Rubbery polymer     100                                                       α-methylstyrene homopolymer                                                                 as specified                                              Carbon black        60                                                        Stearic acid        1                                                         Tackifier           4                                                         MBTS                1.3                                                       Zinc Oxide          3                                                         Sulphur             0.5                                                       ______________________________________                                    

Static peel adhesion of the vulcanizates was tested using a modifiedversion of the Pirelli Peel Test. In this procedure, a small slab of therubber compound to be tested is placed in face to face contact with asimilar slab of a control rubber compound, with a small tefloninterface. Fabric layers are placed on both remote surfaces, and theassembly is cured, by heating for 30 minutes at 166° C. Then the forceper unit-width required to pull the components apart at their mutualinterface is measured at 100° C. and reported, in kiloNewtons per meter.When the adhesion is high, the separation sometimes occurs by tearingwithin the body of the rubber slabs under test. This is referred to as"stock tear", and represents a maximum value for the adhesion. In thepresent case, the control rubber compound against which the testcompounds were cured was a natural rubber compound, representative oftruck tire carcass compounds.

The actual compounds used and the test results on the vulcanizates areprovided in Table II. Sample 1 in Table II is a control formulationcontaining 7 parts by weight per 100 parts by weight rubber ofhydrocarbon extender oil in place of the α-methylstyrene homopolymerresin. The test results indicate that loadings of α-methylstyrenehomopolymer, up to 15 parts by weight, result in marked improvements inair impermeability and marked reductions in mill shrinkage. The othertest results indicate that vulcanizates derived from the rubbercompositions of this invention possess the necessary balance of physicalproperties which renders them useful as a replacement for the controlformulation.

                                      TABLE II                                    __________________________________________________________________________                    Sample #                                                                      1  2  3  4  5  6  7  8  9  10                                 __________________________________________________________________________    Glass transition                                                                              -- 21 21 21 56 56 56 70 70 70                                 temperature of resin, °C.                                              Amount of resin added, phr                                                                    -- 5  10 15 5  10 15 5  10 15                                 COMPOUND PROPERTY                                                             Compound Mooney 57 69 67 65 66 68 67 69 70 70                                 (ML 1 + 4 at 100° C.)                                                  Mill Shrinkage, %                                                                             19 14 14 12 17 15 13 16 14 11                                 VULCANIZATE PROPERTY                                                          Hardness, Shore A                                                                             52 63 66 66 68 69 70 65 66 68                                 100% Modulus, MPa                                                                             1.2                                                                              1.5                                                                              1.4                                                                              1.4                                                                              1.7                                                                              1.6                                                                              1.6                                                                              1.6                                                                              1.7                                                                              1.7                                300% Modulus, MPa                                                                             4.3                                                                              4.9                                                                              4.1                                                                              3.7                                                                              5.0                                                                              4.6                                                                              4.0                                                                              5.1                                                                              4.8                                                                              4.5                                Tensile Strength, MPa                                                                         10.3                                                                             9.5                                                                              8.8                                                                              8.2                                                                              9.5                                                                              9.2                                                                              8.3                                                                              9.9                                                                              9.4                                                                              8.7                                Elongation, %   740                                                                              700                                                                              730                                                                              745                                                                              690                                                                              730                                                                              740                                                                              690                                                                              710                                                                              675                                AGED PROPERTY                                                                 (168 hrs at 120° C.)                                                   Hardness, Shore A                                                                             66 72 75 76 72 75 75 72 73 75                                 100% Modulus, MPa                                                                             2.5                                                                              3.1                                                                              3.0                                                                              3.2                                                                              3.1                                                                              3.1                                                                              3.0                                                                              3.2                                                                              3.2                                                                              3.2                                300% Modulus, MPa                                                                             7.1                                                                              7.7                                                                              6.8                                                                              6.6                                                                              7.5                                                                              7.0                                                                              6.5                                                                              7.8                                                                              7.2                                                                              6.8                                Tensile Strength, MPa                                                                         9.4                                                                              8.8                                                                              7.9                                                                              7.7                                                                              8.3                                                                              7.7                                                                              7.2                                                                              8.5                                                                              8.0                                                                              7.5                                Elongation, %   520                                                                              470                                                                              460                                                                              460                                                                              400                                                                              400                                                                              400                                                                              410                                                                              400                                                                              390                                Permeability to air (pre-                                                                     3.2                                                                              2.5                                                                              2.5                                                                              2.3                                                                              2.4                                                                              2.4                                                                              2.3                                                                              2.3                                                                              2.0                                                                              1.9                                conditioned 24 hrs at room                                                    temp. and 50 psi) at 65° C.,                                           × 10.sup.-8                                                             Static Peel Adhesion (to                                                                      11.3                                                                             19.6                                                                             20.8                                                                             18.1                                                                             11.0                                                                             11.0                                                                             11.0                                                                             10.3                                                                             18.8                                                                             20.3                               100% natural rubber carcass)                                                  at 100° C., kN/m                                                       __________________________________________________________________________

EXAMPLE 2

Using the compounding procedure described in Example 1, ten chlorobutylpolymer compounds were prepared according to the general recipe shown inTable I, in which the chlorobutyl polymer is the same as that describedabove.

The resulting vulcanizates were tested for tensile properties, hardness,air permeability, static peel adhesion and cut growth properties asdescribed previously. The actual compounds used and the results of thesetests are provided in Table III. Sample 11 in Table III is a controlformulation containing 7 parts by weight per 100 parts by weight rubberof hydrocarbon extender oil in place of the α-methylstyrene homopolymer.These test results indicate that along with an improvement in airimpermeability and static peel adhesion there also exists a desirablebalance of other physical properties.

                                      TABLE III                                   __________________________________________________________________________                    Sample #                                                                      11 12 13 14 15 16 17 18 19 20                                 __________________________________________________________________________    Glass transition                                                                              -- 21 21 21 56 56 56 70 70 70                                 temperature of resin, °C.                                              Amount of resin added, phr                                                                    -- 5  10 15 5  10 15 5  10 15                                 COMPOUND PROPERTY                                                             Compound Mooney 59 68 73 66 74 73 72 75 74 77                                 (ML 1 + 4 at 100° C.)                                                  Mill Shrinkage, %                                                                             23 15 15 15 19 17 17 14 14 12                                 VULCANIZATE PROPERTY                                                          Hardness, Shore A                                                                             56 61 63 61 60 65 65 61 63 63                                 100% Modulus, MPa                                                                             1.4                                                                              1.4                                                                              1.3                                                                              1.3                                                                              1.7                                                                              1.6                                                                              1.5                                                                              1.5                                                                              1.5                                                                              1.5                                300% Modulus, MPa                                                                             5.2                                                                              4.7                                                                              4.3                                                                              4.1                                                                              5.8                                                                              5.1                                                                              4.7                                                                              5.1                                                                              4.7                                                                              4.5                                Tensile Strength, MPa                                                                         10.1                                                                             9.8                                                                              9.3                                                                              8.8                                                                              9.6                                                                              9.2                                                                              8.5                                                                              10.4                                                                             9.6                                                                              9.0                                Elongation, %   650                                                                              700                                                                              705                                                                              700                                                                              630                                                                              620                                                                              680                                                                              680                                                                              710                                                                              680                                AGED PROPERTY                                                                 (168 hrs at 120° C.)                                                   Hardness, Shore A                                                                             62 67 67 68 67 66 70 68 69 71                                 100% Modulus, MPa                                                                             1.9                                                                              2.7                                                                              2.6                                                                              2.6                                                                              2.3                                                                              2.2                                                                              2.3                                                                              2.6                                                                              2.7                                                                              2.8                                300% Modulus, MPa                                                                             6.7                                                                              8.4                                                                              7.5                                                                              7.6                                                                              7.4                                                                              6.6                                                                              6.3                                                                              8.3                                                                              7.9                                                                              7.3                                Tensile Strength, MPa                                                                         10.5                                                                             10.1                                                                             9.1                                                                              9.2                                                                              10.1                                                                             9.9                                                                              8.9                                                                              10.6                                                                             9.9                                                                              8.8                                Elongation, %   550                                                                              420                                                                              390                                                                              420                                                                              500                                                                              520                                                                              520                                                                              420                                                                              420                                                                              410                                Permeability to air (pre-                                                                     2.9                                                                              2.2                                                                              2.1                                                                              2.2                                                                              2.0                                                                              2.0                                                                              2.0                                                                              2.4                                                                              2.4                                                                              2.0                                conditioned 24 hrs at room                                                    temp. and 50 psi) at 65° C.,                                           × 10.sup.-8                                                             Static Peel Adhesion (to                                                                      1.0                                                                              1.2                                                                              1.3                                                                              1.3                                                                              1.2                                                                              1.2                                                                              1.4                                                                              1.2                                                                              1.2                                                                              1.8                                100% natural rubber carcass)                                                  at 100° C., kN/m                                                       __________________________________________________________________________

EXAMPLE 3

Using the compounding procedure described in Example 1, four bromobutylpolymer compounds were prepared according to the general recipe shown inTable IV, in which the bromobutyl polymer is the same as that describedabove.

The resulting vulcanizates were tested for tensile properties, hardness,air permeability and static peel adhesion as described previously. Theresults of these tests are provided in Table V. These test resultsindicate that when hydrocarbon extender oil is deleted from the recipe,the resulting compounds exhibit a reduction in mill shrinkage while thecorresponding vulcanizates exhibit an improvement in both airimpermeability and static peel adhesion.

                  TABLE IV                                                        ______________________________________                                                          Parts by weight                                             ______________________________________                                        Bromobutyl polymer  100                                                       α-Methylstyrene homopolymer                                                                 15                                                        (Tg = 70° C.)                                                          Carbon black        60                                                        Oil                 as specified                                              Stearic acid        1                                                         Tackifier           4                                                         MBTS                1.3                                                       Zinc oxide          3                                                         Sulphur             0.5                                                       ______________________________________                                    

                  TABLE V                                                         ______________________________________                                                          Sample #                                                                      21   22     23     24                                       ______________________________________                                        Amount of oil added, pbw                                                                          7      4      2    0                                      COMPOUND PROPERTY                                                             Compound Mooney (ML 1 + 4 at                                                                      56     62     66   72                                     100° C.)                                                               Mill Shrinkage, %   19     19     17   16                                     VULCANIZATE PROPERTY                                                          Hardness, Shore A   63     63     66   70                                     100% Modulus, MPa   1.4    1.5    1.5  1.6                                    300% Modulus, MPa   3.6    3.8    4.1  4.5                                    Tensile Strength, MPa                                                                             8.6    8.6    8.6  8.9                                    Elongation, %       755    725    710  695                                    AGED PROPERTY                                                                 (168 hrs at 120° C.)                                                   Hardness, Shore A   68     73     73   75                                     100% Modulus, MPa   2.9    3.2    3.1  3.5                                    300% Modulus, MPa   6.0    6.5    6.8  7.1                                    Tensile Strength, MPa                                                                             6.9    7.2    7.4  7.4                                    Elongation, %       420    410    390  350                                    Permeability to air (precon-                                                                      3.2    2.3    2.5  2.2                                    ditioned 24 hrs at room temp.                                                 and 50 psi) at 65° C., × 10.sup.-8                               Static Peel Adhesion (to 100%                                                                     15.2   15.7   15.0 16.7                                   natural rubber carcass) at                                                    100° C., kN/m                                                          ______________________________________                                    

EXAMPLE 4

Using the compounding procedure described in Example 1, three bromobutylpolymer compounds were prepared according to the recipes shown in TableVI, in which the bromobutyl polymer is the same as that described above.Samples 26 and 27 in the above mentioned Table contained a mixture ofbromobutyl polymer and α-methylstyrene homopolymer that was solutionblended (100 parts by weight bromobutyl polymer plus 15 parts by weightα-methylstyrene homopolymer) prior to compounding.

The resulting vulcanizates were tested for tensile properties, hardness,air permeability and static peel adhesion as described previously, andalso for tear strength according to ASTM D624-81 and for fatigue lifeusing the Monsanto "fatigue to failure" (FTF) test. In this test,dumbbell shaped pieces of cured rubber, of standard size, are gripped bytheir ends in the jaws of a test machine, and repeatedly stretched andrelaxed. The number of stretching cycles to breaking of the samples isrecorded, and averaged over twelve test samples. In this case, thesamples were repeatedly stretched to 140 percent extension by the use ofcam #24. The results of these tests are provided in Table VII. Sample 25in Table VII is a control formulation. These test results indicate thatwhen bromobutyl polymer and α-methylstyrene homopolymer are solutionblended prior to compounding, an improvement in air impermeability andstatic peel adhesion is observed over the control formulation.

                  TABLE VI                                                        ______________________________________                                                          25    26      27                                            ______________________________________                                        Bromobutyl polymer  100     --      --                                        Bromobutyl polymer/ --      115     115                                       α-methylstyrene homopolymer                                             solution blend                                                                Carbon black        60      60      60                                        Oil                 7       2       --                                        Stearic acid        1       1       1                                         MBTS                1.3     1.3     1.3                                       Tackifier           4       4       4                                         Zinc oxide          3       3       3                                         Sulphur             0.5     0.5     0.5                                       ______________________________________                                    

                  TABLE VII                                                       ______________________________________                                                             SAMPLE #                                                                      25   26     27                                           ______________________________________                                        COMPOUND PROPERTY                                                             Compound Mooney        59     68     69                                       (ML 1 + 4 at 100° C.)                                                  Scorch at 135° C.                                                                             12.8   12.0   9.2                                      (5 pt rise), min.                                                             VULCANIZATE PROPERTY                                                          Hardness, Shore A      60     66     67                                       100% Modulus, MPa      1.1    1.4    1.4                                      300% Modulus, MPa      4.0    3.6    3.6                                      Tensile strength, MPa  8.9    6.9    6.5                                      Elongation, %          680    700    640                                      AGED PROPERTIES (168 hrs at 120° C.)                                   Hardness, Shore A      67     75     76                                       100% Modulus, MPa      2.3    2.8    2.9                                      300% Modulus, MPa      6.6    6.0    6.8                                      Tensile strength, MPa  8.9    6.2    7.0                                      Elongation, %          510    340    350                                      Static Peel Adhesion (to 100%                                                                        10.8   14.7   15.7                                     natural rubber carcass) at                                                    100° C., kN/m                                                          Monsanto Flex to Failure (Cam #24)                                            unaged, kc             179    283    449                                      aged (168 hrs at 120° C.) kc                                                                  27     12     14                                       Tear strength                                                                 at room temp., kN/m    40     30.9   30.4                                     at 100° C., kN/m                                                                              36.3   31     27.9                                     Permeability to air (precon-                                                                         3.2    2.0    2.0                                      ditioned 24 hrs at room temp.                                                 and 50 psi) at 65° C., ×  10.sup.-8                              ______________________________________                                    

EXAMPLE 5

Using the compounding procedure described in Example 1, four butylpolymer compounds were prepared according to the general recipe shown inTable VIII, in which the butyl polymer is the same as that describedabove.

The resulting vulcanizates were tested for tensile properties, hardness,air permeability and static peel adhesion properties as describedpreviously. The results of these tests are provided in Table IX. Sample28 in Table IX is a control formulation containing 7 parts by weight per100 parts by weight rubber of hydrocarbon extender oil in place of theα-methylstyrene homopolymer. These test results indicate that along withan improvement in air impermeability, there also exists a desirablebalance of other physical properties.

                  TABLE VIII                                                      ______________________________________                                                          Parts by weight                                             ______________________________________                                        Butyl polymer       100                                                       α-Methylstyrene homopolymer                                                                 as specified                                              (Tg = 70° C.)                                                          Carbon black        60                                                        Stearic acid        1                                                         Tackifier           4                                                         MBTS                1.3                                                       Zinc oxide          4                                                         Sulphur             1.5                                                       TMTD                1                                                         ______________________________________                                    

                  TABLE IX                                                        ______________________________________                                                         SAMPLE #                                                                      28   29      30     31                                       ______________________________________                                        Amount of resin added, phr                                                                       --     5       10   15                                     COMPOUND PROPERTY                                                             Compound Mooney    72     89      87   83                                     (ML 1 + 4 at 100° C.)                                                  Mill Shrinkage, %  21     17      17   17                                     VULCANIZATE PROPERTY                                                          Hardness, Shore A  62     72      70   70                                     100% Modulus, MPa  2.0    2.6     2.6  2.7                                    300% Modulus, MPa  5.5    6.2     6.1  4.8                                    Tensile Strength, MPa                                                                            12.2   10.1    10.6 8.6                                    Elongation, %      620    580     600  640                                    AGED PROPERTIES                                                               (168 hrs at 120° C.)                                                   Hardness, Shore A  63     72      72   80                                     100% Modulus, MPa  1.4    1.8     1.8  1.8                                    300% Modulus, MPa  3.9    4.3     4.3  3.6                                    Tensile Strength, MPa                                                                            8.4    6.8     6.5  5.0                                    Elongation, %      705    605     510  550                                    Permeability to air (pre-                                                                        3.7    2.5     2.5  2.4                                    conditioned 24 hrs at                                                         room temp. and 50 psi)                                                        at 65° C., × 10.sup.-8                                           Static Peel Adhesion (to 100%                                                                    0.01   0.01    0.01 0.02                                   natural rubber carcass) at                                                    100° C., kN/m                                                          ______________________________________                                    

What is claimed is:
 1. A new rubber composition, the vulcanizate ofwhich has reduced permeability to gases, comprising:one hundred parts byweight of at least one rubbery polymer selected from the groupconsisting of butyl rubber, chlorinated butyl rubber and brominatedbutyl rubber, from about 3 to about 20 parts by weight ofα-methylstyrene homopolymer having a softening point of from about 93°C. to about 150° C. and a Tg of from about 15° C. to about 75° C., fromabout 30 to about 90 parts by weight of at least one carbon black, from0 to about 7 parts by weight of hydrocarbon extender oil, and a curingsystem.
 2. The composition of claim 1, wherein the amount ofα-methylstyrene homopolymer is from about 12 to about 18 parts byweight.
 3. The composition of claim 2, wherein the amount of carbonblack is from about 50 to about 70 parts by weight.
 4. The compositionof claim 3, wherein said curing system comprises from about 1 to about10 parts by weight of zinc oxide, from about 0.2 to about 2 parts byweight of elemental sulphur and from about 0.5 to about 3 parts byweight of at least one sulphur based accelerator.
 5. The composition ofclaim 4, wherein the amount of said zinc oxide is from about 2 to about5 parts by weight.
 6. The composition of claim 5, wherein thehydrocarbon extender oil is paraffinic.
 7. The composition of claim 6,wherein the amount of hydrocarbon extender oil is from 0 to about 4parts by weight.
 8. The composition of claim 7, wherein the halogenatedbutyl rubber is chlorinated butyl rubber.
 9. The composition of claim 7,wherein the halogenated butyl rubber is brominated butyl rubber. 10.Vulcanizates of the composition of claim
 8. 11. Vulcanizates of thecomposition of claim
 9. 12. A pneumatic tire having an inner linercomprising the vulcanizate of claim
 10. 13. A pneumatic tire having aninner liner comprising the vulcanizate of claim
 11. 14. A membranematerial comprising the vulcanizate of claim
 10. 15. A membrane materialcomprising the vulcanizate of claim
 11. 16. The composition of claim 7,wherein the α-methylstyrene homopolymer has a softening point of fromabout 130° to about 150° C. and a Tg of from about 65° C. to about 75°C., further characterized in that no hydrocarbon extender oil is used.17. The composition of claim 16, wherein the halogenated butyl rubber isbrominated butyl rubber.
 18. Vulcanizates of the composition of claim17.
 19. A pneumatic tire having an inner liner containing thevulcanizate of claim
 18. 20. A process for producing a vulcanizatehaving reduced permeability to gases which comprises:(A) mixing onehundred parts by weight of at least one rubbery polymer selected fromthe group consisting of butyl rubber, chlorinated butyl rubber andbrominated butyl rubber with an α-methylstyrene homopolymer, whereinsaid α-methylstyrene homopolymer is used in an amount of from about 3 toabout 20 parts by weight and has a softening point of from about 93° C.to about 150° C. and a Tg of from about 15° C. to about 75° C., fromabout 30 to about 90 parts by weight of at least one carbon black, from0 to about 7 parts by weight of hydrocarbon extender oil, and a curingsystem, (B) forming the mixture produced in (A) into a sheet, and (C)vulcanizing the sheet formed in (B) at a temperature of from about 150°C. for from about 5 to about 60 minutes.
 21. The process of claim 20,wherein the amount of α-methylstyrene homopolymer is from about 12 toabout 18 parts by weight.
 22. The process of claim 21, wherein therubbery polymer is brominated butyl rubber.
 23. The process of claim 22,wherein the α-methylstyrene homopolymer has a softening point of fromabout 110° C. to about 150° C. and a Tg of from about 50° C. to about75° C.
 24. The process of claim 22, wherein the α-methylstyrenehomopolymer has a softening point of from about 130° C. to about 150° C.and a Tg of from about 65° C. to about 75° C.